General help message cleanup

Many of the help messages were not really helpful; for example, many
commands that take no arguments would not print a correct synopsis
line, but "No additional help available." which is not exactly wrong,
but not helpful either.

Commit ``Make "usage" messages more helpful.'' changed this
partially. But it also became clear that lots of "Usage" and "Help"
messages (fields "usage" and "help" in struct cmd_tbl_s respective)
were actually redundant.

This patch cleans this up - for example:

Before:
	=> help dtt
	dtt - Digital Thermometer and Thermostat

	Usage:
	dtt         - Read temperature from digital thermometer and thermostat.

After:
	=> help dtt
	dtt - Read temperature from Digital Thermometer and Thermostat

	Usage:
	dtt

Signed-off-by: Wolfgang Denk <wd@denx.de>
diff --git a/board/sandburst/karef/karef.c b/board/sandburst/karef/karef.c
index b8ae68f..92f5247 100644
--- a/board/sandburst/karef/karef.c
+++ b/board/sandburst/karef/karef.c
@@ -611,7 +611,7 @@
 }
 
 U_BOOT_CMD(kasetup, 1, 1, karefSetupVars,
-	   "Set environment to factory defaults", NULL);
+	   "Set environment to factory defaults", "");
 
 U_BOOT_CMD(karecover, 1, 1, karefRecover,
-	   "Set environment to allow for fs recovery", NULL);
+	   "Set environment to allow for fs recovery", "");